Okay, I admit it: as a Southern speaker, I’m horrible about using these two words correctly; we almost always say “further” instead of “farther” around here. (In fact, “farther” sounds like a strange, alien word to me.) Maybe it happens because people don’t know the difference between the two, or it’s just that the pronunciation (for us) of “farther” and “further” are so close, it’s hard to tell them apart, so they’ve blended into one word.
Whatever the reason, it not the sort of grammatical mistake you want to make in your writing. Grammarians will pull their hair and gnash their teeth at you.
So here’s the way to tell them apart:
Farther equates to distance. “The runner pushed himself farther than he had ever been.” This is simple to remember, since everyone knows the root word, far, means distant. If you (or your character) are going off into the distance, you need to go FARther to get to it.
Further, this bloggant saith not.